This invention relates to electronic communication systems, and in particular to wireless multiple-access communication systems.
Many technological systems have more than one operational state, with transitions between operational states triggered by changes in one or more parameters or conditions. In order to avoid a “trigger-happy” system, a so-called time-to-trigger (TtT) parameter is often provided to delay a state transition after a parameter change. For example, the TtT parameter can see to it that a state change does not occur unless the system has been in a steady state for at least a period of time, i.e., the time to trigger. The hysteresis introduced into the operation of the system by the TtT parameter helps prevent the system from “ping-ponging” between states.
Such TtT parameters are used by user equipments (UEs), such as mobile telephones and other remote terminals, in various wireless communication systems, including cellular radio telephone systems like the universal mobile telecommunications system (UMTS). The UMTS is a third generation (3G) mobile communication system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The UMTS employs wideband code division multiple access (WCDMA) for the air interfaces between UEs and base stations (BSs) in the system. The Third Generation Partnership Project (3GPP) promulgates specifications for the UMTS and WCDMA systems. This application focuses on WCDMA communication systems simply for economy of explanation, and the artisan will understand that the principles described in this application can be implemented in other communication systems.
FIG. 1 depicts a mobile radio cellular telecommunication system 10, which may be, for example, a WCDMA communication system. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. More generally, each RNC directs UE calls via the appropriate BSs, which communicate with each UE through downlink (DL) and uplink (UL) channels. RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26. Each BS, which is a Node B in 3GPP parlance, serves a geographical area that can be divided into one or more cell(s). BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26. The BSs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Both RNCs 12, 14 are connected with external networks, such as the public switched telephone network (PSTN), the Internet, etc., through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
As a UE moves with respect to the BSs, and possibly vice versa, an on-going connection is maintained through a process of hand-off, or handover, of the connection from one BS to another BS. Early cellular systems used hard handovers (HHOs), in which a first BS (covering the cell that the UE was leaving) would stop communicating with the UE just as a second BS (covering the cell that the UE was entering) started communication. Modern cellular systems, including UMTS systems, typically use diversity, or soft, handovers (SHOs), in which a UE is connected simultaneously to two or more BSs. The multiple radio links operating simultaneously are sometimes called the “active set”. In FIG. 1, UEs 28, 30 are shown communicating with plural BSs in diversity handover situations. UE 28 communicates with BSs 16, 18, 20, and UE 30 communicates with BSs 20, 22. A control communication link between the RNCs 12, 14 permits diversity communications to/from the UE 30 via the BSs 20, 22. According to the current standards, a UE may be simultaneously connected to up to six BSs in SHO, which is to say that the UE may have as many as six cells in its active set.
The network (NW), e.g., the RNCs and BSs, grants and sets up the SHO and generally controls the cells in a UE's active set based on cell-quality reports sent to the NW by the UE. A cell-quality report can be based on UE-measurements of the average signal-to-interference ratio (SIR) of control channels, such as common pilot channels (CPICHs), of all cells in the UE's active set and other cells that the UE receives. The UE measures the SIRs on a regular basis (typically five times per second). It will be understood that cell-quality reports can also be based on measurements of other parameters, e.g., received signal code power (RSCP).
When the UE determines that a new cell has a SIR that is better than the SIR of a cell in the active set, an Active Set Update—ADD procedure is initiated that is described, for example, at Sections 8.3.4 and 14.11 of 3GPP TS 25.331 V 5.19.0, Radio Resource Control (RRC) Protocol Specification (Release 5) (December 2006). The UE reports Event 1a (Radio Link Addition) to the NW, and an RNC informs the new BS to start uplink (UL) synchronization. After an acknowledgement message from the new BS is received in the RNC, an “Active Set Update—ADD” message is transmitted to the UE, and the new BS starts to transmit on the DL to the UE.
To avoid a prematurely adding the new cell to the active set, the UE does not report Event 1a unless the new cell has had a better SIR, i.e., the new cell has fulfilled the necessary triggering condition, for at least a certain period of time, which is to say that the new cell has been better for at least a TtT period of time. The TtT is typically a parameter that the UE receives from the NW. In general, the UE starts a timer when a new cell's measured quality fulfils the requirements described in TS 25.331 cited above and stops the timer when that cell's measured quality drops below the aforementioned requirements.
This operation of the UE is depicted in FIG. 2, which is a plot of measured cell quality on the vertical axis against time on the horizontal axis. The measured quality of a first cell in the active set is indicated by the solid line, and the measured quality of a second cell not in the active set but monitored by the UE is indicated by the dashed line. The UE sets and resets its TtT timer as the cell qualities vary with respect to each other. As indicated by the vertical line, the UE starts its timer when the measured quality of the second cell exceeds the measured quality of the first cell, and as indicated by the vertical arrow, the UE reports Event 1a to the NW when the time period measured by the timer has elapsed, which is to say when the measured quality of the second cell has exceeded the measured quality of the first cell for the TtT period.
It will be appreciated that there are many other applications of a time-to-trigger parameter in communication and other systems. For example, a TtT parameter is used in a similar manner for other measurement events reported in WCDMA communication systems as described in 3GPP TS 25.331. It will also be appreciated that the name of the parameter need not always be “time-to-trigger”. For example, Section 5.2.6.1 of 3GPP TS 25.304, UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode (Release 5) (September 2005) describes parameters called Treselection and Penalty_Time, which are used as TtT parameters.
U.S. Pat. No. 7,082,301 to Jagadeesan et al. describes a method for handing off a call between networks that includes handing off the call when the quality of a first link is less than a handoff trigger threshold for a “drop count” duration and when the quality of a second link is greater than a minimum quality threshold. As described in column 9, for example, the handoff trigger thresholds are used to prevent ping-ponging.
U.S. Patent Application Publication No. US 2006/0258386 by Jeong et al. describes a ping-pong duration threshold and a ping-pong occurrence number threshold, and a UE that uses a non-scaled down cell reselection time limit in a non-high-speed UE state, and a scaled-down cell reselection limit in a high-speed UE state.
Although steady-state guard times like a TtT period can avoid ping-pong effects and trigger-happy systems, such fixed guard times can cause problems. For example, when it comes to tuning the value of such a guard time, there is always a trade-off between system stability and system response lag. The longer the guard time is, the more stable the system is (state changes are less frequent), but also the slower the system responds to a stimulus. Conversely, the shorter the guard time is, the faster the system responds to a stimulus, but also the more prone the system becomes to ping-ponging (state changes are more frequent).
In the example of a UE's reporting Event 1a, a too-long TtT period slows down the handover process, but a too-short TtT leads to quality-measurement reports flooding the NW. A NW operator usually tries to tune the TtT parameter to avoid both of those negative behaviors, but it is impossible to find a value that adequately covers all possible cases.
For example, consider a UE that is located on the border of a cell and is moving out of the cell when the UE tries to set up a call or request a service. If the UE succeeds in setting up a connection to the cell, the UE may experience a bad connection/reception because the UE is moving out of the cell. As the connection/reception worsens, the UE might find a much better quality neighboring cell, but the UE is not allowed to trigger an event to inform the NW about the new cell unless the TtT period has elapsed. Eventually the UE can lose its connection to the NW despite the fact that good cells exist in its vicinity. As a result of the loss of a call/service in such scenarios, the UE vendor and the NW operator lose goodwill.
In addition, problems with TtT periods have from time to time led to problems with acceptance tests of equipment by operators. Problems arising out of TtT periods can also be seen in urban areas where cells suddenly appear and disappear as a result of complex cell planning.